Remove preloading of the first file in the diff viewer.

The diff viewer preloaded the first file in an attempt to reduce
perceived load time by ensuring that upon the first load, there was
something to review.

This is great in theory, but it doesn't really survive some of the new
work. When we reach the point of being able to offload diff generation,
we won't be able to ensure the first file is loaded, creating an
inconsistent experience when enabling such a feature.

Also, some of the new review UI code gets much more complex with
preloading, since we need to handle some logic in two different places.

So, this rips that out. We'll need to get a feel for the loading time
perception of this, and see if there are any decent workarounds.
Add review UIs to the diff viewer.

We now display review UIs in the diff viewer for file attachments that
have an inline-capable review UI. This takes the place of the thumbnail
that we would otherwise render.

When displaying an inline-capable review UI, the "Review" button goes
away, and the review UI is just rendered inline. It can be commented on,
just like normal.

At the moment, both the original and modified versions each have a
review UI capable of commenting. It's unclear whether the left-hand side
should allow commenting on a review UI, but right now, it's there. Extra
work would need to be done to tell the AbstractReviewables that they're
not allowed to add commenting capabilities.

The next step is to add diffable file attachment support to review UIs.